Application of electronic devices using liquid crystals is not limited to watches, clocks, thermometers, and other similar devices. It is expected that such electronic devices find wider applications including word processors, laptop computers, and even TV receivers.
Various kinds of liquid crystals are available. Among others, nematic liquid crystals have enjoyed wide acceptance. By contriving the mode of operation, these nematic liquid crystals are used as twisted nematic liquid crystals and supertwisted liquid crystals all of which find extensive application. Such a liquid crystal alone is held between a pair of substrates having a patterned electrode and used as a simple matrix panel. A comparatively small number of steps are needed to manufacture the panel. Also, this panel is economical to manufacture. However, where the number of pixel electrodes is large, crosstalk occurs, thus deteriorating the quality of the displayed image.
To solve this problem, nonlinear devices have been fabricated on substrates from thin-film transistors or metal-insulator film-metal structure at the sacrifice of simplicity of manufacturing process.
These help liquid crystal to switch between different states. As a result, a good-quality picture can be created. Also, clear TV images can be accomplished by liquid crystal panels. However, in the case of nematic liquid crystals, the response speeds are 1 to 500 msec. Their writing speeds are low. Accordingly, ferroelectric and antiferroelectric liquid crystals have attracted attention.
Referring to FIG. 1, liquid crystal molecules 102 of a ferroelectric liquid crystal are oriented in a given direction by the surface of a substrate 100. A layered structure 101 is formed between any adjacent ones of the liquid crystal molecules. These layered structures are arranged highly orderly in three dimensions. It is considered that where the cell is thick, liquid crystal molecules can lie in any positions where cones are formed. They are spirally arranged.
Referring again to FIG. 1, where the cell is thin, the direction of the long axis of each liquid crystal molecule assumes both a first state 102 and a second state 103. The spontaneous polarization of the liquid crystal molecule can be controlled by the direction of an electric field applied to the molecule.
In the first state, the spontaneous polarization is directed downward. In the second state, the spontaneous polarization is directed upward. The spontaneous polarization can be switched between these two states at a high speed. The present state can be maintained stably after cease of the application of the electric field. When observed via a polarizer, the two states can be distinguished over a wide range of viewing angles. Since ferroelectric liquid crystals have excellent features in this way, it is much expected that they act as liquid crystal materials capable of realizing high-speed viewing screens of large capacity. Referring next to FIG. 2, with respect to antiferroelectric liquid crystals, the direction of the long axis of each liquid crystal molecule assumes a first state 120 and a second state 121 in the same way as the aforementioned ferroelectric liquid crystals. In addition, the direction of the long axis of each antiferroelectric liquid crystal molecule can take a third state 122. When no voltage is applied, the third state is assumed. When a negative voltage is applied, the first state is taken up. When a positive voltage is applied, the second state is assumed.
A clear threshold voltage exists between the third and the first states. Also, a clear threshold voltage exists between the third and second states. The presence of these threshold voltages makes it much more easier to drive antiferroelectric liquid crystals than ferroelectric liquid crystals.
The speeds at which the liquid crystal is switched from the third to first state and from the third to second state are increased with increasing the applied voltage. However, the speeds at which the state is switched from the first to third state and from the second to third state tend to decrease somewhat possibly because the viscosity of the liquid crystal and the interfaces affect the switching action relatively greatly and because a sufficient force to change the directions of spontaneous polarizations from a uniform direction to alternating directions does not exist.
All of these characteristics of antiferroelectric liquid crystals are similar to the characteristics of nematic liquid crystals rather than those of ferroelectric liquid crystals.
Both ferroelectric liquid crystals and antiferroelectric crystals form layered structures. These layers are not perpendicular to the substrate surfaces but bent to some extent and form a V-shaped structure. Where such a layered structure is normal to the substrate surface, it is referred to as the bookshelf structure. Where the structure is bent, it is referred to as the chevron structure. Since the layers can be bent in two directions, a defect is created at the interface between two adjacent layers bent in different directions.
A liquid crystal cell can be easily observed with a polarization microscope. This defect deteriorates the capability to retain information and the contrast ratio, which in turn makes it impossible to display a good-quality image. One method of solving this problem is to use an orientation film of a high pretilt angle so that a uniform bending direction is obtained. Another method is to use a liquid crystal material which maintains the layers vertical to the substrate surfaces. A further method is to apply an electric field, for changing the chevron structure to the bookshelf structure. All of these are difficult techniques, and it is difficult to suppress defects.
However, in the case of an antiferroelectric liquid crystal, layers are easily deformed by an electric field. The layers bent like the letter "V" are made vertical to the substrate surfaces. Consequently, good orientation free of defects can be accomplished. An antiferroelectric liquid crystal itself has a threshold value. In addition, it can be oriented well. Hence, antiferroelectric liquid crystals can be handled with greater ease than ferroelectric liquid crystals.
In this way, ferroelectric and antiferroelectric liquid crystals have similar characteristics and different characteristics. In both kinds of liquid crystals, two states are uniquely determined and so it is difficult to change the gray level by the applied voltage unlike in the case of a twisted nematic liquid crystal. Thus, it has been considered that it is difficult to produce various gray levels from ferroelectric and antiferroelectric liquid crystals.
As a result, neither ferroelectric liquid crystals nor antiferroelectric liquid crystals have been used in displays which have high switching speeds and wide viewing angles and still require a wide gray scale such as a TV screen. Accordingly, there is an urgent demand for techniques capable of fabrication of displays using a ferroelectric or antiferroelectric liquid crystal and achieving a wide gray scale.
The typical waveforms of pulse signals used to drive an actual ferroelectric liquid crystal and the response of the liquid crystal are now described by referring to FIG. 3. This ferroelectric liquid crystal is driven by a simple matrix consisting of plural electrodes. One of the pulse signals is a select pulse 131 having a large voltage for selecting the state of the liquid crystal. The other is a non-select pulse 132 having a voltage that is one third or one fourth of the voltage of the select pulse 131.
The liquid crystal is optically switched from a bright state 133 (e.g., a first state) to a dark state 134 (a second state) in response to the select pulse. Then, the liquid crystal responds to the non-select pulse. It is unlikely that the liquid crystal goes back to the first state from the second state. However, optical fluctuations take place. This is a major cause of a decrease in the contrast ratio.
Under this condition, it is impossible to maintain an intermediate state between the first and second states even if the pulse height of the select pulses is changed, for the following reason. Positive and negative voltages are alternately applied to pixel electrodes and so the amount of electric charge is not kept constant but rather varies constantly. Hence, the optical response is not stable.
Where the simple matrix is driven in this way, the problem is whether liquid crystal molecules can certainly assume first and second states and whether a high-contrast ratio state can be obtained. It has been impossible to achieve a gray scale stably.
Accordingly, where a ferroelectric liquid crystal is driven with a simple matrix to produce a gray scale, most techniques take plural (n) pixels as a single picture element displayed on the display device. Various gray levels are created by producing various ratios of the area of ON pixels to the area of OFF pixels. This area ratio gray scale scheme can produce 2.sup.n gray levels. However, in order to provide a given area of display, the number of pixels on the display device is required to be n times as many as the number of pixels conventionally required. Furthermore, the display creates a rough picture and, therefore, it is impossible to create a high-resolution picture. In this way, in order to accomplish a higher-resolution display, it is necessary to realize plural gray levels within each one electrode pixel.
The transmittance and the refractive index of a liquid crystal material are affected by an external electric field. By making use of this property, an electrical signal can be converted into an optical signal. In consequence, a display can be provided. Known liquid crystal materials are twisted-nematic (TN) liquid crystals, supertwisted-nematic (STN) liquid crystals, ferroelectric liquid crystals, and antiferroelectric liquid crystals. In recent years, polymer dispersed liquid crystals (PDLC) comprising high polymers in which nematic, ferroelectric, or antiferroelectric liquid crystals are dispersed have come to be known. It is known that a liquid crystal does not respond to an external voltage in an infinitely short time but rather a given time passes until the liquid crystal responds to the voltage. The time is intrinsic to each individual liquid crystal material. The time is tens of milliseconds for twisted-nematic liquid crystals, hundreds of milliseconds for supertwisted-nematic liquid crystals, tens of microseconds for ferroelectric liquid crystals, and tens of milliseconds for polymer dispersed liquid crystals utilizing nematic liquid crystals.
Of display devices making use of liquid crystals, those devices which use the active-matrix structure produce the best image quality. Conventional liquid crystal electro-optical devices of the active-matrix type use thin-film transistors (TFTs) as active devices. The TFTs are fabricated from an amorphous or polycrystalline semiconductor. TFTs of only one type, or either P- or N-type, are used for each one pixel. In particular, N-channel TFTs (NTFTs) are connected in series with each one pixel. Signal voltages are applied to signal lines arranged in rows and columns. When both vertical and horizontal signal voltages are applied to a TFT located at the junction of a horizontal signal line and a vertical signal line, the TFT is activated. In this way, individual liquid crystal pixels are separately activated and deactivated. A liquid crystal electrooptical device showing a large contrast can be accomplished by controlling pixels in this manner.
However, it has been very difficult for the prior art active-matrix type to create a gray scale including halftones or color tones. A method utilizing the fact that the transmittance of a liquid crystal varies according to the applied voltage has been discussed to realize a gray scale. More specifically, an appropriate voltage is applied between the source and drain of each one of TFTs arranged in rows and columns from a peripheral circuit. Under this condition, a signal voltage is applied to the gate electrode to apply the same voltage to the corresponding liquid crystal pixel.
In this method, voltages actually applied to liquid crystal pixels differ by at least several percent because the TFTs or matrix lines are not homogeneous. On the other hand, the dependence of the transmittance of a liquid crystal on the voltage shows quite strong nonlinearity, and:the transmittance varies rapidly at a given voltage. Therefore, even if two pixel voltages differ by only a few percent, their transmittances may differ greatly. For this reason, the prior art analog gray scale display method can achieve only 16 gray levels at best. For example, in a TN liquid crystal material, a transitional region where the transmittance varies has a width of only 1.2 V. To attain 16 gray levels, it is necessary to control a quite small voltage of 75 mV. This has reduced the production yield greatly.
The aforementioned difficulty in realizing a gray scale display has made liquid crystal displays much less competitive than conventional CRTs which have enjoyed wide acceptance. We have found that a gray scale can be obtained visually by controlling the time for which a voltage is applied to a liquid crystal. This technique is described in detail in Japanese Patent application Ser. No. 169306/1991.
For example, where a twisted-nematic liquid crystal which is a typical liquid crystal material is used, brightness can be varied by applying various pulse waveforms to liquid crystal pixels, as shown in FIG. 11. That is, the brightness can be increased in a stepwise fashion in the order 1, 2, . . . , 15 shown in FIG. 11. In the example of FIG. 11, an image can be displayed at 16 gray levels. For instance, in FIG. 11(A), a pulse having a duration of 1 unit is applied at gray level 1. A pulse having a duration of 2 units is applied at gray level 2. A pulse having a duration of 2 units and a pulse having a duration of 1 unit are applied at gray level 3. As a result, a pulse having a duration of three units is applied. A pulse having a duration of 4 units is applied at gray level 4. A pulse having a duration of 1 unit and a pulse having a duration of 4 units are applied at gray level 5. A pulse having a duration of 2 units and a pulse having a duration of 4 units are applied at gray level 8. Furthermore, a pulse having a duration of 8 units is prepared. As a result, a pulse having a duration of 15 units can be obtained.
Specifically, 2.sup.4 =16 gray levels can be produced by appropriately combining 4 kinds of pulses, i.e., pulses having durations of 1 unit, 2 units, 4 units, and 8 units, respectively. If more kinds of pulses such as pulses having durations of 16 units, 32 units, 84 units, and 128 units, respectively, are prepared, then a gray scale having more gray levels such as 32 gray levels, 64 gray levels, 128 gray levels, and 256 gray levels, can be derived. For example, in order to obtain 256 gray levels, 8 kinds of pulses should be prepared.
In the example of FIG. 11(A), the duration of the voltage applied to each pixel increases in geometrical series, such as T.sub.1, 2T.sub.1, 4T.sub.1, and so forth. The duration may also be varied from T.sub.1 to 8T.sub.1 and then to 2T.sub.1 and finally to 4T.sub.1, as shown in FIG. 11(B). This arrangement can reduce the burden imposed on a unit which transfers data to a display unit.
However, where a TN liquid crystal is used, the accuracy of the voltage applied must be as high as the accuracy of the prior art analog gray scale display method. Specifically, when "10" shown in FIG. 11 is displayed by applying 5 V to a pixel to activate it, the obtained brightness is darker by about 2% than the brightness obtained by applying 5.1 V to activate the pixel so as to display the same "10". That is, in this digital gray scale display method, TFTs are required to have uniform characteristics in the same way as the prior art analog gray scale display method.
FIG. 13(A) is a circuit diagram of a typical TFT active-matrix circuit. Variations in the potential V.sub.1 on a liquid crystal pixel caused by applying a scanning signal V.sub.G and a data signal V.sub.D are shown in FIG. 13(B).
Some factors contribute to the variations in the potential V.sub.1. A major factor is a potential drop V produced when the scanning signal is interrupted due to parasitic capacitances on the gate electrode of each TFT and on leads extending from the pixel electrode. Another major factor is a voltage drop caused by a leakage current from the TFT and by a leakage current from the liquid crystal. Where TFTs cannot be driven sufficiently, i.e., the mobility is small, if a sufficient electrical charging cannot be done during a time t.sub.1 for which the scanning signal persists, then the reached voltage becomes nonuniform, thus causing the above-described variations.
These variations are affected materially by the characteristics of TFTs and so if the TFTs differ widely in characteristics, then individual pixels differ greatly in brightness. For instance, if the parasitic capacitances on the gate electrodes and the parasitic capacitances on the leads extending from the pixel electrodes are not uniform, then the voltage drops .DELTA.V differ. If the leakage currents from the TFTs differ, then pixel voltages decrease at various speeds. In the case of TFTs having a low mobility such as amorphous silicon TFTs, nonuniform electrical charging also presents problems. For these reasons, even if the same signal is applied, the pixel potential V.sub.1 may show either a characteristic curve as indicated by the solid line in FIG. 13(B) or a characteristic curve as indicated by the broken line. Of course, such variations in characteristics are not desirable.